Extra Property is a geometric parameter by which two similar elements differ with all other properties being the same. In most cases, these parameters affect cost in a cost estimation report.
Open the Breakdown Manager and you will see that your model has a traditional Revit Breakdown structure (Category - Family - ElementType) by default. Extra Properties are placed right after ElementType.
Eg. One parameter by which two types of ceilings differ is the thickness (20 mm for the first group and 30 mm for the second one).